Reduction of iron-containing ores

ABSTRACT

A process for the reduction of iron ores and ores of other metals containing iron such as ilmenite using a high volatile non-caking coal as the reductant and fuel characterized in that part of the coal is added at the discharge end of the kiln in such a manner that it is distributed along the length of the kiln to within the kiln feed end zone of the kiln and so that substantially no coal is incorporated in the bed within at least the last 15% of the kiln length, and in that the remainder of the coal is added at the feed end of the kiln.

United States Patent 1 Hockin June 17, 1975 REDUCTION OF IRON-CONTAINING ORES Harry William Hockin. Cape], Australia Related U.S. Application Data [63] Continuation of Ser No. [90.67], Oct. 19, NHL

[75] Inventor:

3.663,2Ul 5/]972 Heitmann l. 75/36 X Primary E.tuniirierL Dewayne Rutledge Assistant ExuminerM. J. Andrews [57] ABSTRACT A process for the reduction of iron ores and ores of other metals containing iron such as ilmenite using a high volatile non-caking coal as the reductant and fuel abandoned. characterized in that part of the coal is added at the discharge end of the kiln in such a manner that it is [52] U.S. Cl 75/33; 75/36 distributed along the length of the kiln to within the [5]] Int. Cl C2lb 13/08 kiln feed end zone of the kiln and so that substantially [58] Field of Search 75/33, 34, 36 no coal is incorporated in the bed within at least the last l5% of the kiln length, and in that the remainder [56] References Cited of the coal is added at the feed end of the kiln.

UNlTED STATES PATENTS 5 Claims, No Drawings $505,060 4/l970 Heitmann 75/36 AIR way 0R5 VULA 7/45 l A/fl/VM/(l/VG l-I/GH C041. VOLJTILE NONCIK/A/G EEOC/C T/OA/ K/L/V can REDUCED 1 REDUCTION OF IRON-CONTAINING ORES This application is a continuation of Ser. No. 190,671, filed Oct. 19, 1971, now abandoned.

This invention relates to a process for the reduction of iron ores and ores of other metals containing iron such as ilmenite. it is particularly applicable to the reduction of fine ores which are difficult to reduce, notably ilmenite and titanifcrous magnetite or titanifcrous hematite derived from alluvial or beach deposits, in which the largest ore particle size is not greater than 0.50 mm. and the fine particles may be as small as 0.040 mm. diameter, in a rotary kiln which is provided with air-injection pipes or shell burners and using countercurrent flow of gas and charge.

The ease of reduction of titaniferous ores relative to that of hematite or magnetite has been referred to in the literature with the conclusion that the more titanium there is in the ore, the more difficult it is to reduce. Thermodynamic data for the reduction of ferrous titanite with hydrogen, carbon monoxide, and carbon show that the equilibria are less favorable than for the corresponding reduction of ferrous oxide. llmenite for instance, is more stable than ordinary iron oxides because of the combination of iron oxide with titanium dioxide, the combination being associated with a reduction in free energy. The greater diifficulty in reducing ilmeniite can thus be understood. Unfortunately this fact has not been always fully taken into account in processes involving reduction, and the problem is further complicated by the fact that the titaniumbearing minerals such as ilmenite vary very considerably in their mineralogical composition and the effect of this is not always appreciated, as such variation in the number and type of initial phases present can very considerably influence any subsequent reduction. A particular case is that of ilmenite from heavy mineral sand deposits which often undergoes natural alteration whereby a progressive breakdown of the ilmenite lattice occurs to give a material which is either an amorphous iron titanium oxide or a mechanical mixture of an amorphous titanium oxide and iron oxide, followed by recrystallization of this amorphous substance to give finely crystallized rutile.

Such mixtures react to reduction differently from pure ilrnenite, and in fact may be no more difficult to reduce than hematite or magnetite. However, many ilmenites which have not undergone natural alteration can present considerable difficulty in reduction.

Also, most materials reduced in rotary kilns are of a coarse particle size, or if the original material is fine, have been pelletized or otherwise aggregated. In lump ore processing the average grain size generally ranges from 5 to l5 mm., or even up to mm. for readily reducible ores. When pellets are used, grain sizes of approximately lOl5 mm. are preferred. Although some application to fine ores has been achieved, sizes less than about I 2 mm. would not normally be considered as suitable for such a process.

Various methods have been suggested for operating a rotary kiln using a non-caking high volatile coal as heating agent and reductant, whereby the coal is fed into the kiln through the discharge end by mechanical or pneumatic means in such a manner that it is distributed throughout the width of the surface of the charge. These methods are suitable for the achievement of 90-95% metallization of iron in easily reducible ores,

but it has found that they will not achieve more than metallization on, say, unaltered ilmenite. Also with fine ores it is found that only part of the ore charge is likely to be reduced to the optimum degree There are considerable disadvantages in blowing all of the high volatile coal into the kiln at the discharge end and there are also disadvantages in feeding such coal in at about the center of the kiln as is claimed in some literature. Because air is supplied to the kiln at a constant rate the composition of the chamber gas is subject to fluctuation so that the reducing and combustion processes are not uniform and the control of the process is adversely affected. When the reducing agent used has a high content of volatile matter or moisture. such as is the case with many low grade coals, the pressure in the rotary kiln is also subject to general and local fluctuations which further affect the control of the process, and which lead to a non-uniform discharge of solids from the kiln. As the distribution of the coal throughout the kiln is so highly critical, addition of all the coal at the discharge end makes the process difficult to control for simple mechanical reasons.

We have now found that, contrary to the known processes, with a combination of coal feeding at the feed end of the kiln and of coal feeding at the discharge end of the kiln, provided that certain requirements in regard to the discharge end feed system are met, it is possible to obtain a high degree of metallization consistently and that more than metallization can be achieved with a difficulty reducible ore such as fine unaltered ilmenite.

Normally, feeding of a high volatile coal at the feed end of a countercurrent flow system leads to a loss of volatile material in the first kiln section. These volatiles are thus lost to the process and increase the heat value of the waste gas, and only a portion of the gases from the low temperature distillation can be used for the process. The volatiles in the waste gas burn due to the inevitable access of air from outside the kiln and can cause high temperatures and operating difficulties in the dust collecting chambers and waste gas system.

On the other hand, feeding all the fuel and reductant requirements in the form of a high volatile coal at the discharge end of the kiln has also been found to have certain disadvantages, as in spite of the benefits in regard to fixed carbon consumption derived from this technique, the control of the operation can be very difficult due to the large amount of fuel and reductant that has to be fed, and the need to have a highly precise distribution of the fuel if a high degree of reduction is to be achieved. It has been found that in practice this fuel distribution is not possible to maintain and consequently variable reduction results. It has also been found that the incorporation of high volatile coal into the kiln bed at the discharge end of the kiln results in impaired reduction capability in the kiln due to variations in the COICO ratio in the bed and in the chamber gas, and that this situation tends to limit the degree of metallization to a level below that required.

These difficulties can be overcome by feeding a portion of the coal at the discharge end of the kiln, sufficient to control the temperature profile throughout the kiln, and by insuring that this coal is distributed in the kiln in such a manner that substantially no coal lands in the final reducing zone of the kiln. Furthermore the coal should be fed in in such a manner that a portion of it is distributed almost to the feed end of the kiln. In

this manner the full benefit of the remaining portion of the coal fed at the feed end is obtained.

Thus the present imention resides in a process for the reduction of iron ore and ore of other metals containing iron in a rotary kiln fitted with air injection dcvices using a high \olatile non-caking coal as the reductant and fuel characterized in that part of the coal is added at the discharge end of the kiln in such a manner that substantially no coal is incorporated in the kiln bed within at least the last I50. and preferably the last of the kiln length and so that some of the coal added at the discharge end of the kiln is distributed to within the feed end Zone of the kiln. and in that the remainder of the coal is added at the feed end of the kiln.

The invention also resides in apparatus for reducing iron ore or ore of other metals containing iron comprising a rotary kiln fitted with air injection devices. means to introduce coal at the discharge end of the kiln in such manner that substantially no coal lands within at least the last 159% of the length of the kiln nearer the discharge end and so that some of the thus introduced coal is distributed to within the feed end zone of the kiln. and means to introduce further coal at the feed end of the kiln.

The invention will be explained in greater detail here inafter with reference to the accompanying drawing in which the sole figure is a flow diagram illustrating the process to which the invention applies.

The amount of coal fed into the kiln at the discharge end in accordance with the invention to maintain a satisfactory and controllable temperature profile is preferably within the range of 20 by weight ofthe total coal feed. The coal may be fed by conventional pneu matic techniques and the rate of feed and the particle size of the coal is controlled to insure that the condi tions referred to above are obtained. A typical satisfactory coal distribution obtained using pneumatic feeding is shown below:

ature in the zone where it lands and adversely affects reduction in that zone. An even coal distribution improves char cover over the bed. Dropping the coal within the last 207r of the kiln has the following effects:

I. Marked reduction reversals can occur in the dis charge end of the kiln.

2. Erratic surging in the kiln discharge is apt to occur.

3. Reoxidation can occur in the cooler. This to gether with the possible combustion of volatiles in the cooler results in uncontrollable cooler discharge temperatures of up to 300C.

4. A high level of char consumption may result.

5. An unsatisfactory temperature profile is obtained.

The feeding of a small amount of coal into the dis charge end of the kiln as described above also has an unexpected result in controlling the amount of volatile gases in the waste gas discharge and it is possible to reduce the volatile content of the waste gas to about 2%. This enables the temperature of the waste gases to be effectively controlled.

High volatile non-caking coals which are suitable for the purposes of the present invention are those which on heating produce more than 25% by weight of volatiles and the particles of which do not fuse together to form coke but rather produce a char. Coal from the Collie coalfields of Western Australia is one example of a suitable high volatile noncaking coal.

A problem may arise in rotary kilns due to insufficient mixing of the charge which has an important effect in reducing the heat transfer into the ore in the charge. High kiln rotational speeds are often cited as necessary to obtain satisfactory mixing, but this has the disadvantage of introducing mechanical problems and also results in reduced residence time which may require changes in the kiln geometry to achieve the required residence time. This mixing problem is particularly severe in the case of fine (less than 0.50 mm. di-

Size Distribution of Coal At Each Percentage Point in Kiln.

75 Distance Front fl W'l' Distribution Discharge End of Kiln of Total Coal Fed I27 6.35 3.34 I40 I00 I00 at Discharge End mm. mm. mm. mm. mm. mm.

5 NIL l0 NIL l5 0.7 1.7 5.1 25.1 36.2 8.8 22.1 21 7.] L0 5.0 25.8 36.4 7.4 24.4 27 10.9 0.4 2.6 33.7 45.9 7.0 l0.4 33 10.7 4.0 I I3 43.4 34.7 1.0 5.6 39 I33 4.0 13.0 5|.8 20.5 I0 I06 3 7.3 30.2 53.6 8.7 0| ()2 51 10.4 9.3 38.0 50.5 2.0 NIL 0.2 57 10.4 2.2 57.3 39.5 0.8 NIL 0.2 63 5.2 27.4 44.2 27.8 0.4 NIL 02 69 4.3 25.3 54.7 V9.5 0.4 NIL 0.1 75 4.7 3|.l 50.9 17.4 0.4 NIL 02 XI 34 l) 3 70.7 9.6 4,0 NIL NIL 87 3.6 32.2 62.2 5.0 0.4 NIL 02 93 5 5 64.8 32.5 2.4 0.2 NIL 0.l

COAL FEED SIZE DISTRIBUTION I45 19.7 27.0 I84 3.6 16.8

The coal feed rate should be controlled within close limits, say :1 i570 variation. Unsteady feeding results in variation in the degree of reduction. erratic kiln discharge rates and large temperature and pressure tluctuations within the kiln. The coal should also be distrib uted as evenly as possible along the length of the kiln. Coal landing in a heap causes a marked drop in temper ameter) dense (greater than 8.0. 4.0) ore such as ilmenite from alluvial deposits, or hematite ore pro duced by conventional concentration methods from certain types oforc body. and has largely prevented the application of rotary kilns to the reduction of such materials without some form of prior pelletizing or aggregatron.

When the ore feed to the kiln is mixed with a quantity of char, preferably recycle char as shown in the drawing. obtained from previous operation of the kiln. in the usual way. it has been found that when treating a fine ore less than 0.50 mm. largest particle size there is a tendency for only part of the ore to be reduced in the kiln due to poor mixing which often results from the combination of fine heavy ore and the slightly coarser light char reductant. It has been found. however. that satisfactory mixing can be obtained during reduction if the ore to char ratio is controlled within closely defined limits, and that in this manner a high degree of reduction can be obtained and maintained. At the same time a constant ratio of reduced ore to char is maintained in the discharge material, which overcomes any problems of performance of subsequent equipment such as coolers, magnetic separators. or screens.

The preferred weight ratio of reduced ore to char in the discharge material is within the range of 5 of reduced ore to 1 of char to of reduced ore to l of char, the preferred ratio for fine ores being 10 of reduced ore to l of char. At ratios of reduced ore to char greater than this range. reoxidization of the reduced ore may occur, or the ore charge may become sticky and form lumps and accretions within the kiln. At lower ratios. surging of the kiln discharge material may occur with variations in the total discharge tonnage and in the ratio of ore to char in the discharge. Under these latter circumstances it will be found that only part of the ore is satisfactorily reduced, the remainder being only partially reduced often at a very low level.

The following example will serve to illustrate the in vention:

A rotary kiln 30 metres in length and 2.4 metres outside diameter was fed with coal, oxidized ilmenite, and recycle char of the compositions and size analyses listed below.

Approximately 60% of the length of the kiln was maintained at a temperature of ll80C 1200C by control of air injected through tubes in the kiln. The coal was the only fuel added.

COAL (a) Composition 2 22.8% Volatile Matter 31.09? Fixed Carbon 43.5% Ash 2.7% S 0.43; Fe 0.27% (b) Size Analysis Plus 12.7 mm 107% By wt. Plus 6.35 mm. Minus 12.7 mm 20.7% do. Plus 3.34 mm. Minus 6.35 mm 29.6% do. Plus 1.40 mm, Minus 3.34 mm 19.5% do. Plus 1.00 mm, Minus 1.40 mm 3.1% do. Minus 1.00 mm 16.4% do. OXlDlZED lLMENlTE (a) Composition Ti0 55.1% FeO 7.5% Fe O 34.0)? Fe [Total] 29.6% (b) Size Analysis Plus 0.30 mm 0.6% By wt. Plus 0.21 1 mm, Minus 0.30 mm 8.79% do. Plus 0.152 mm. Minus 0.211 mm 44.7; do. Plus 0.105 mm. Minus 0.152 mm 422; do. Minus 0.105 mm 3.8% do.

RECYCLE CHAR Composition -Continued F B C Remainder lb) Size Analysis Plus ll7 min 1.6% B wt Plus 6.35 min. Minus 12.7 mm 1.6") do Plus 3.34 mm. Minus 6.35 mm 159) do. Plus 1.40 mm. Minus 3.34 mm 5' .li do. Plus 1.00 mm. Minus 1.40 mm 14.1'4 do Plus 0.70 mm. Minus 1.00 mm 12 4% do. Minus 1.00 mm 2.3) do.

The oxidized ilmenite was fed to the kiln at the rate of 2.000 Kg/hour. together with the recycle char at the rate of 400 Kg/hour at the feed end of the kiln. Coal of the above composition was also fed at the feed end of the kiln, at the rate of 1.000 Kg/hour. and a further 400 Kg/hour was injected at the discharge end in the manner described above. After stabilization of operations. the discharging reduced ilmenite was sampled every two hours over a 24 hour period with the following results.

It is thus clearly demonstrated that a high degree of metallization can be obtained with a fine ore such as ilmenite.

While the invention has been described with particular reference to the reduction of ilmenite, it is equally applicable to the reduction of other finely divided metal ores containing iron, and to finely divided iron ores. The invention is particularly applicable to the reduction of ores having an average particle size of less than 0.5 mm in diameter.

1 claim:

1. A process comprising reducing fine ore containing iron in a rotary kiln fitted with air injection devices. and having a discharge end, a feed end zone and a bed, said process further comprising using a high volatile noncaking coal as the reductant and the fuel, and adding part of the coal at the discharge end of the kiln in such a manner that substantially no coal is incorporated in the bed within at least the last 20% of the kiln length and so that some of the coal added at the discharge end of the kiln is distributed to within the feed end zone of the kiln, the remainder of the coal being added at the feed end of the kiln, the amount of coal fed into the kiln at the discharge end being about 20-30% by weight of the total coal feed, said iron being metallized to at least about 2. A process comprising reducing fine ore containing iron in a rotary kiln fitted with air injection devices, and having a discharge end. a feed end zone and a bed. and using a high volatile non-caking coal as the reductant and the fuel, said process further comprising adding part of the coal at the discharge end of the kiln in such 7 a manner that it is distributed along the length of the kiln to within the feed end zone of the kiln and so that substantially no coal is incorporated in the bed within at least the last 207 of the kiln lengtl the remainder of the coal being added at the feed end of the kiln. the amount of coal fed into the kiln ut the discharge end being about 20-30% by weight of the tot-til coal l'eed. said iron being metullized to ill least about 95%.

3. A process as claimed in claim 2. in which the ore 

1. A PROCESS COMPRISING REDUCING FINE ORE CONTAINING IRON IN A ROTARY KILN FITTED WITH AIR INJECTION DEVICES, AND HAVING A DISCHARGE END, A FEED END ZONE AND A BED, SAID PROCESS FURTHER COMPRISING USING A HIGH VOLATILE NON-CAKING COAL AS THE REDUCTANT AND THE FUEL, AND ADDING PART OF THE COAL AT THE DISCHARGE END OF THE KILN IN SUCH A MANNER THAT SUBSTANTIALLY NO COAL IS INCORPORATED IN THE BED WITHIN AT LEAST THE LAST 20% OF THE KILN LENGTH AND SO THAT SOME OF THE COAL ADDED AT THE DISCHARGE END OF THE KILN IS DISTRIBUTED TO WITHIN THE FEED END ZONE OF THE KIL THE REMAINDER OF THE COAL BEING ADDED AT THE FEED END OF THE KILN, THE AMOUNT OF COAL FED INTO THE KILN AT THE DISCHARGE END BEING ABOUT 20-30% BY WEIGHT OF THE TOTAL COAL FEED, SAID IRON BEING METALLIZED TO AT LEAST ABOUT 95%.
 2. A process comprising reducing fine ore containing iron in a rotary kiln fitted with air injection devices, and having a discharge end, a feed end zone and a bed, and using a high volatile non-caking coal as the reductant and the fuel, said process further comprising adding part of the coal at the discharge end of the kiln in such a manner that it is distributed along the length of the kiln to within the feed end zone of the kiln and so that substantially no coal is incorporated in the bed within at least the last 20% of the kiln length, the remainder of the coal being added at the feed end of the kiln, the amount of coal fed into the kiln at the discharge end being about 20-30% by weight of the total coal feed, said iron being metallized to at least about 95%.
 3. A process as claimed in claim 2, in which the ore feed to the kiln is mixed with recycled char.
 4. A process as claimed in claim 3, in which the weight ratio of reduced ore to char in the discharge material is within the range of 5 of reduced ore to 1 of char to 15 of reduced ore to 1 of char.
 5. A process as claimed in claim 3, in which the said weight ratio is 10 of reduced ore to 1 of char. 